FIELD OF THE INVENTION
[0001] This invention relates generally to the field of integrated circuits and more particularly
to the design of leadframes for use with encapsulation packaging of integrated circuits,
and specifically to the use of leadframes without dambars in transfer molding processes.
BACKGROUND OF THE INVENTION
[0002] In producing integrated circuits, it is desirable to provide packaged integrated
circuits having plastic, epoxy or resin packages which encapsulate the die and a portion
of the lead frame and leads. These packages have been produced using a variety of
methods, a few of which will be described here.
[0003] Conventional molding techniques take advantage of the physical characteristics of
the mold compounds. For integrated circuit package molding applications, these compounds
are typically thermoset compounds. These compounds consist of an epoxy novolac resin
or similar material combined with a filler, such as alumina. Other materials such
as accelerators, curing agents, fillers, and mold release agents are added to make
the compound suitable for molding.
[0004] The transfer molding process as known in the prior art takes advantage of the viscosity
characteristics of the molding compound to fill cavity molds containing the die and
leadframe assemblies with the mold compound, which then cures around the die and leadframe
assemblies to form a solid, coherent, package which is relatively inexpensive and
durable, and a good protective encapsulation for the integrated circuit.
[0005] Transfer molding operations have three stages which correspond to the three phases
of viscosity of thermoset mold compounds.
[0006] First there is a preheat stage required to move the mold compound from its hard initial
state to the low viscosity state. Second is a transfer stage, where the compound is
low in viscosity and easily transported and directed into cavities and runners. This
transfer process should be rapid and should be completed before the mold compound
begins to set. Finally there is a cure stage that occurs following the transfer stage.
Thermoset compounds are heat cured. Other compounds may not require heat to cure,
such as thermoplastics.
[0007] Conventional transfer molding techniques also require leadframes which include dambars.
FIG. 1 depicts a conventional lead frame assembly and package. In FIG. 1, lead frame
21 is depicted, and package 23 is shown cutaway from the top of the lead frame 21.
The leads 25 are shown coupled together outside of the outer edge of package 23 by
dambar 33. Die pad 29 is positioned to receive the integrated circuit die and support
the die. Die pad support strap 31 is used to keep the die and support pad 29 planar
during the die attach, bonding and molding operations.
[0008] FIG. 2 depicts in a cross sectional view a lead frame and die assembly in a prior
art transfer mold cavity. Leadframe 21 is placed in between a top mold chase 41 having
a top mold cavity 43 and a bottom mold chase 45 having a bottom mold cavity 47. Primary
runner 49 is coupled to secondary runner 51 and to gate 53. Integrated circuit die
55 is shown positioned on the die support pad of lead frame 21.
[0009] In operation, the mold compound is typically heated to attain a low viscosity state.
The mold compound is then forced by compression into the primary runner 49 from a
mold pot or other mold compound source, which is not shown. Secondary runner 51 then
routes the mold compound over gate 53 into the cavity formed by the top and bottom
cavities 43 and 47. The mold compound is forced through the narrow gate 53 into the
cavity until the cavity fills with mold compound to form the encapsulated package
which will have a shape defined by the shape of the top and bottom mold cavities 43
and 47.
[0010] Since the mold cavity is closed around the leadframe 21, and the leads extend outward
beyond the edge of the cavities 43 and 47, there is a space between the leads, not
visible in FIG. 2, where the mold compound can be forced out through the space and
travel out from the cavities between the leads of leadframe 21. The dambar, which
is shown in FIG. 1 but is not visible in FIG. 2, acts as a stop and allows the mold
compound to travel out a short distance, but no farther. This extra mold compound
forms flash between the leads, the flash forming in the area between the edge of the
package and the dambar.
[0011] Therefore, in the prior art transfer molding process that uses a mold cavity to form
a package around the leadframe as shown in FIG. 2, the dambar of FIG. 1 acts as a
physical stop to prevent the mold compound from flowing out from the cavity area of
the mold chases and forming an interlead flash or sprue joint between adjacent leads.
The dambar is required to mold the package. After the package is completed, the conventional
process flow is to remove the metal dambar and the flash, using metal trim and compound
deflash technology. The dambar also provides lead to lead stability during processing
and lead planarity during processing. Once the package is completed and cured, the
dambar is no longer needed for stability and is removed.
[0012] As the number of pins increases to above 200 pins, the pitch between leads is decreasing.
Also, thinner package requirements result in a requirement for thinner leadframes.
So called "fine pitch" leadframes are now required. The lead to lead pitch for a fine
pitch leadframe is typically less than 0.5 millimeters. The conventional dambar approach
is no longer economically satisfactory for the production of fine pitch lead frames,
because the trim and flash steps are so difficult to achieve.
[0013] These thinner lead frames with decreased lead to lead pitch are costly to process.
Increased accuracy and handling precision is required for each step of the process.
Costs of production increase as these requirements increase. Two important production
cost drivers are the dambar trim and interlead flash removal steps. To remove the
dambar and flash after molding for a fine pitch leadframe will require extremely precise
machining during both the trim and flash removal stages of processing. The trim stage
will become extremely difficult and require increasingly accurate and expensive equipment
as the lead to lead pitch continues to decline. The deflash stage is a mechanical
or chemical step which requires either additional precision machining or chemical
processing, which also leads to chemical waste disposal costs.
[0014] Accordingly, a need thus exists for a package encapsulation molding system which
uses a leadframe without a dambar to produce packaged integrated circuits having fine
pitch leadframes. Eliminating the dambar also eliminates the problems and costs of
the prior art trim and deflash processing steps. The molding system should provide
a high part per hour throughput rate, low raw material costs, and be simple to operate,
maintain, and use molding stations that are relatively inexpensive to build. The new
system should be compatible with existing single pot transfer mold presses to allow
a retrofitting of existing integrated circuit assembly lines.
SUMMARY OF THE INVENTION
[0015] A dambarless leadframe and molding system for transfer molding the packages of integrated
circuits having fine pitch leadframes and using mold release films is provided. The
mold compound is packaged as inserts in a plastic packaging that is sealed at the
edges by a removable or reversible seal. As the package is heated in the mold, these
edge seals become flexible. The prepackaged mold compound can then be pushed through
the edge seals at places adjacent to the mold runners during the transfer molding
process. The protective packaging ensures that the mold compound is free from moisture
and air contamination and is easily produced, stored and shipped.
[0016] An improved mold design is used in combination with the mold compound inserts. The
mold chases include rectangular receptacles for receiving the pencil shaped inserts
of prepackaged mold compound. A rectangular plunger is provided for each of the receptacles.
Each package cavity is preferably equidistant from the receptacle containing the mold
compound, providing improved uniformity of fill and allowing for complete fill of
the cavities with reduced wire sweep as compared to the transfer molds of the prior
art. The rectangular plunger is inserted and forced against the prepackaged mold compound,
and the mold compound is forced through the edge seals of the protective packaging
into short runners coupling the mold receptacle to the cavities. The mold compound
then hardens and completes the molding process.
[0017] A release film is preferably used with the mold compound. The mold cavities are covered
with a release film before the leadframe assemblies are placed inside the mold receptacle.
A vacuum is used to stretch the release film into the cavities within the mold chases.
A top and a bottom release film are each stretched over the top and bottom mold chases,
respectively. The mold is closed and the release films are positioned around the cavities
and therefore around the integrated circuit die and die pad. The leads of the leadframe
extend out of the cavity and are positioned between the top and bottom release films.
When the mold is closed and the dambar and release films are compressed together the
two pieces of release film elongate and form an interface that closes the gap that
exists between the leads at the edge of the cavity. As the mold compound is fed into
the cavity, the mold compound falls within the space between the two release films
and encapsulates the die and die pad. The release film now also serves as a barrier
at the edges of the cavity and stops the mold compound from transferring or traveling
outside the cavity, thus eliminating the need for the dambars of the prior art leadframes.
[0018] A dambarless leadframe is provided. The leadframe of the invention is designed to
work with the release film to prevent the mold compound from forming interlead mold
compound flash outside of the cavity area. The dambarless leadframe also includes
additional support means as required to provide the lead planarity and stability previously
provided by the dambar, to ensure the leads remain in place during molding. This support
means may be, in one embodiment, a tape placed inside the package edge area to provide
the needed planarity and stability. Alternatively an adhesive or polymer support may
be provided. The support means is kept at an equal or substantially constant distance
from the inner lead ends to provide improved support. Also, the leadframe leads are
preferably wider at the package edge area to reduce the interlead distance in the
region adjacent to the cavity edge. This reduced interlead distance will reduce the
likelihood that interlead flash will be formed. This reduced interlead distance also
improves the ability of the release films to stop compound flow outside the mold cavities.
An open gate design is used to further enhance the operation of the molding process
and the release films. The leadframe is produced using conventional leadframe production
methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The present invention will now be further described, by way of example, with reference
to the accompanying drawings in which:
FIG. 1 depicts a prior art leadframe having a dambar;
FIG. 2 depicts a cross sectional view of a prior art transfer mold for encapsulating
a lead frame with mold compound;
FIG. 3 depicts a cross sectional view of the release films and leadframe of the invention;
FIG. 4 depicts a second cross sectional view of the release films and leadframe of
the invention;
FIG. 5 depicts a cross sectional view of the application of the release film of the
invention to a bottom mold cavity;
FIG. 6 illustrates the release film stretching process of the invention;
FIG. 7 depicts the various stretched positions of the release film of the invention;
FIG. 8 depicts a package formed with the dambarless leadframe of the invention;
FIG. 9 depicts a prepackaged mold compound insert for use with the invention;
FIG. 10 depicts a cross sectional view of the molding process of the invention using
the prepackaged mold compound, the release films, and the dambarless leadframe of
the invention;
FIG. 11 depicts a top view of the dambarless leadframe of the invention;
FIG. 12 depicts a portion of the dambarless leadframe of FIG. 11;
FIG. 13 depicts a second portion of the dambarless leadframe of FIG. 11; and
FIG. 14 depicts a third portion of the dambarless leadframe of FIG. 11.
[0020] Corresponding numerals are used for corresponding elements in the drawings, unless
otherwise indicated in the text.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] The leadframe of the invention is formed without the dambar of the prior art. The
leadframe may be stamped out, or a pattern and etch process can be used to produce
the leadframes without the dambar, or dambarless leadframes, using production techniques
well known in the prior art. One exemplary leadframe is made from a plated copper
or nickel alloy substrate, such as Alloy 42, which is plated with palladium and then
spot plated with silver to enhance solderability. Other leadframes known to the prior
art may also be used, so long as the dambar is removed and other inventive modifications
described herein are made. The process for producing packaged IC's using the dambarless
leadframe is now described.
[0022] FIG. 3 depicts a cross sectional view of a mold cavity illustrating the process of
transfer molding process using mold compound and release films to encapsulate a dambarless
leadframe in accordance with the invention. In FIG. 3, leadframe leads 61 are shown
positioned between bottom release film 65, which is stretched over bottom cavity 63,
and top release film 67, which is stretched over top cavity 69. The release films
advantageously protect the mold surfaces from the molding compound. Since conventional
molding compound is an abrasive, the longevity of the mold surfaces is enhanced using
the release films 65 and 67. Further, the release films allow the use of molding compounds
which are highly abrasive as compared to conventional resin or resin filled compounds,
and also compounds which do not readily release from the mold surfaces after cure,
such as adhesive compounds. Epoxies and clear epoxies can be used, particularly, to
advantage for photodiode IC's, CCD IC's and other devices which are packaged such
that they can sense or emit light. Conventional molding compounds are required to
release from the mold after cure.
[0023] FIG. 4 illustrates the use of the release films and cavities of FIG. 3 to mold a
package with the dambarless lead frame of the invention. In FIG. 4, top mold cavity
69 and bottom mold cavity 63 are clamped together around leadframe 61. This action
brings the top and bottom release films 65 and 67 in close proximity. The release
films will fall together and form a seam or interface between the leads in spaces
71. This seam prevents the mold compound from being forced out of the mold cavities
and forming interlead flash.
[0024] In operation, the release films 67 and 65 are stretched across the mold cavities
69 and 63 while the mold is open, between runs or "shots". Leadframe 61 is positioned
over the bottom mold cavity 63. The mold is supplied with mold compound in a mold
pot or receptacle which is not shown in FIG. 4. The top and bottom molds are clamped
together so that top cavity 69 is clamped over leadframe 61 and bottom cavity 63,
and the top release film 67 is clamped over the bottom release film 65. A seam forms
in the spaces 71 between the leads of the leadframe 61. The release film stretches
in between these leads due to the heat in the molding process, which makes the film
stretch farther than when it is cold. The mold compound is forced into the cavities
between the top and bottom release films.
[0025] The seam in spaces 71 keeps the mold compound from leaving the cavity and forming
interlead flash between the leads, as would occur in prior art molding processes.
The seam also therefore eliminates the need for the dambar used on prior art lead
frames. Eliminating the flash and the dambar dispenses with the expensive and precise
steps of removing the flash and the dambar after molding. This is especially advantageous
in the production of integrated circuits using fine pitch leadframes, where the tooling
required to remove the dambar and flash is particularly expensive.
[0026] The release films used in the molding process may be of many materials, but the material
chosen should have certain specific characteristics. The release film should have
a melting point higher than that of the mold compound being used and higher than that
of the molding process being used, so it will not melt away during the process. The
release film should not contaminate the mold with residues. The release film should
release from the mold after molding is completed. The release film should not contaminate
the molded package with ions or other harmful substances which would reduce the life
or reliability of the package.
[0027] Also, the release film should be able to stretch in response to the mold compound
pressure, but not break. That is, the release film elongates during the molding process
without tearing. For the package to be formed correctly the release film should keep
the mold compound contained between the top and bottom films 65 and 67. A set of selection
criteria for release films to be used in the process of the invention has been developed.
The release film should also be thick enough to fill in the spaces 71 between the
external leads during molding, so that no interlead flash or splinters of molding
compound form, because if this happens, the flash removal step will have to be performed.
Typically the release film should be 100 microns - 200 microns thick for typical leadframes,
and less for thinner leadframes such as extremely fine pitch leadframes. The top and
bottom release films may be of different thicknesses. The thickness required for optimal
performance will vary somewhat with the thickness, type, and interlead spacing of
the particular leadframe being molded.
[0028] One type of release film which meets these criteria is known commonly as ETFE, and
contains tetrafluoroethylene and ethylene monomer units. The film is a flouroplastic
material. Such films are available from a variety of sources. One particular film
that gives good results is Hostaflon
™ ET 6235J and Hostaflon™ ET 6210 J, available form Hoescht High Chem, Hoeschst Aktiengesellschaft,
Marketing Hostaflon, D-6230 Frankfurt am Main 80, Frankfurt, Germany. The ETFE film
has a shear strength of 9 N/mm
2 at 200 degrees Celsius, elongation strength of 6 N/mm
2 at 200 degrees Celsius, and a melting point of 260-270 degrees Celsius. This material
meets all of the requirements defined for the process of the invention. Other materials
which meet these requirements may be used, and the advantages of the invention will
be obtained.
[0029] The process shown in FIGS. 3 and 4 will provide packages formed with dambarless leadframes,
however, a number of problems have been identified in production which require additional
process steps to enhance yield. The release films 65 and 67 in FIGS. 3 and 4 can sometimes
interfere with the bondwires and with the entry of mold compound into the cavities.
It has been discovered that by prestretching the release films and then applying a
vacuum in the cavities before the lead frames are introduced, the films 65 and 67
can be pulled into the cavities and will therefore not interfere with either the leadframes
or the mold compound as it flows into the cavities. This vacuum molding process provides
a means to use the release films 65 and 67 to achieve reliable, uniform package shapes
and release, and also to enhance the control of the interlead flash such that no deflash
or trim steps are needed.
[0030] FIG. 5 depicts the release film supply mechanism and the bottom mold 68, and illustrates
the first stretching step required to prepare the bottom release film. In FIG. 5,
film supply mechanism 81 provides a source of the bottom release film. Film takeup
mechanism 83 takes up the used release film in a continuous spool. The release film
65 is fed over the bottom mold 68 and cavity 63 by the combined operation of the supply
mechanism 81 and the takeup mechanism 83.
[0031] In operation, after a molded part is removed from the bottom cavity, film takeup
mechanism 83 pulls away the used release film 65 and automatically indexes a new segment
of the continuous film supply over the bottom cavity 63, the new film being supplied
by supply mechanism 81. By setting the friction in the supply mechanism 81, it is
possible to control the amount of stretching that occurs as the film is taken up and
indexed by film takeup mechanism 83. In this way, the film is stretched as it is pulled
over the bottom cavity 63.
[0032] A similar operation is used for the top die cavity and release films, which is not
shown here for simplicity.
[0033] After the prestretching step shown in FIG. 5, the film 65 will be positioned within
the bottom mold cavity 63, so it provides clearance away from the top and center of
the cavity. This is to ensure that during the molding process the mold compound stays
within the release film and does not get underneath it, and to make sure the packages
are all uniform in shape. A vacuum system is used to pull the release film 65 into
the bottom mold cavity 63.
[0034] FIG. 6 depicts the vacuum system of the invention. In FIG. 6, bottom mold 90 is shown.
Bottom die cavity 63 is formed from cavity block 95, which forms the bottom part of
the die cavity, and cavity inserts 93, which form the sides of the die cavity 63.
Vacuum line 91 is used to draw a vacuum out of the die cavity 63. Vacuum holes 99
are formed at the top lip of the die cavity in the cavity inserts 93, and attract
and hold the release film 65. Vacuum ports 97 are formed at the bottom of the die
cavity 63 to cause the release film to stretch into the cavity and towards the walls
formed from cavity inserts 93. Slot 101 is used to form the rest of the vacuum supply
and is coupled to the vacuum ports 97 by columns 103, and to vacuum supply line 91.
The vacuum ports 97 are formed by a gap positioned between the top of cavity block
95 and the inserts 93, the gap then joining a larger gap which is column 103. The
ports 97 are typically about 0.05- 0.1 millimeters wide.
[0035] In operation, the release film 65 is positioned above the cavity 63 and prestretched
as described above. The vacuum holes 99 and vacuum ports 97 of FIG. 6 are then activated.
The vacuum holes 99 act as anchors which hold that portion of the film 65 which lies
above the holes 99, and allows the remaining portion of the release film 65 that is
above the die cavity 63 to be stretched against the anchor points. The film is stretched
by the vacuum into the cavity and towards the bottom corners of the bottom die cavity
63.
[0036] The top die cavity is similarly configured, and operates in the same manner as the
bottom die cavity and vacuum. Again, for simplicity the top die cavity is not shown
in detail.
[0037] After the film is positioned and stretched into the cavities, the lead frames and
dies are positioned and the mold is closed, as shown above in FIGS. 3 and 4. Since
the release film is positioned out of the way, the mold compound can now flow into
the cavities without interference and without processing errors that result without
the stretched film. The mold compound then stretches the film into its final position,
which pushes the film to the walls of the top and bottom cavities as shown in FIG.
3.
[0038] FIG. 7 depicts the three stretching positions of the release film in the vacuum molding
system inside the mold cavities. In FIG. 7, bottom cavity 63 is positioned in a clamped
position against top cavity 69. Top release film 67 is clamped against bottom release
film 65, as shown in FIGS. 3 and 4. Vacuum supply lines 91 are used to provide vacuum
to bottom cavity 63. Vacuum supply lines 115 are used to provide vacuum to top cavity
69. Gate 111 is shown for supplying the cavity formed by the top and bottom cavities
69 and 63 with mold compound during molding. Reference numerals 117, 119 and 121 indicate
three position lines. Line 117 illustrates the position of the release films when
vacuum is first applied. Line 119 illustrate the position of the release films after
the films are stretched in response to the vacuum. Line 121 indicates the position
of the release films after the molding compound is supplied to the cavity and stretches
the films out to the cavity surfaces of top and bottom cavities 63 and 69. Anchor
point 123 indicates the vacuum hose where the release films are clamped so that they
can be stretched into the cavity. So the top and bottom release films are stretched
in three phases; first to line 117 when the vacuum is first applied to the cavities
63 and 69, then to line 121 in response to the vacuum, and finally to line 121 and
to the cavity surfaces when the mold compound is compressed into the cavity.
[0039] The process as shown thus far will result in satisfactory dambarless lead frames
for certain lead frames which have fine pitch leads and have small interlead spacings.
However, in some instances additional modifications to the lead frame design are required
to achieve adequate or complete elimination of interlead flash. Using the release
films as shown in FIGS. 3 and 4 with the vacuum of FIGS. 6 and 7 will result in the
release films filling the spaces between the leads and stopping the mold compound
at the package edge for most leads. Occasionally, the mold compound will escape the
release films at the seam or interface and form interlead flash in the form of splinters.
Additional changes to the dambarless lead frame are preferable to eliminate the interlead
splinters so that no deflashing is required after molding.
[0040] FIG. 8 depicts a portion of an improved lead frame design for use in the dambarless
molding system of the invention. In FIG. 8, leads 131 are shown extending out from
the edge 133 of package 135. The leads are wider at the clamping edge of the top and
bottom mold cavities to reduce the interlead spacing distance at the clamping point
to less than 0.20 mm.
[0041] Preferably, the spacing distance at the package edge or clamping point should be
less than 0.15 mm. Reducing the interlead spacing distance at this point improves
the seam at the interface between the top and bottom films during the molding process,
and thereby eliminates the interlead splinters which result if the distance is not
reduced. The modification to the lead frame design is straightforward and may be done
using conventional means by the lead frame manufacturer.
[0042] Additional modifications are preferred to replace the stability and lead planarity
support previously provided by the dambars of the prior art leadframes. An internal
tape support system is used as one alternative. Another alternative is to provide
a liquid polymer material which can be dispensed at the internal ends of the lead
fingers and then cured to provide the required stability. The lead frame manufacturer
can perform these steps when the leadframes are produced.
[0043] When the modified dambarless leadframes with tape are used in the film release vacuum
molding system of the invention, packages are produced which advantageously do not
require removal of dambars or removal of interlead flash using deflashing steps. Occasionally,
however, a package defect occurs because the mold compound enters the cavity under
the bottom film. In a preferred embodiment, the mold compound is supplied in a prepackaged
plastic wrapper. The plastic wrapper is used in the molding process to direct the
mold compound to enter the cavities above the bottom release film, therefore the defects
caused by the mold compound getting beneath the release film are reduced or eliminated.
[0044] FIG. 9 depicts a mold compound insert 151 packaged in a plastic film. In FIG. 9,
top 153 is positioned over the mold compound 155 and sleeve 157. Ends 159 are sealed
by end seals 161. Edge 163 is sealed to top 153. Top 153 forms a lip that is wider
than the mold compound 155 to provide support for holding the packaged mold compound
in a mold receptacle. The size of the mold compound insert is determined by the mold
design and by the volume and type of mold compound used.
[0045] The packaged mold compound insert 151 is preferably produced as followed. Mold compound
is provided in a rectangular shape, although other shapes are feasible depending on
the mold design. The mold compound is placed inside sleeve 157 within a support stand
or block. Top film 153 is placed over the mold compound piece 155 and the sleeve 157.
In a first embodiment, heat sealing is used to seal the top 153 to the sleeve 157.
In operation, the edge seal 163 should remain impenetrable during normal storage and
handling, but to open and to allow the mold compound to pass through the seal and
into the runners during molding. A heat seal has been shown to meet these requirements,
but alternative seals such as ultrasonic, adhesive, tape, crimping, and epoxy seals
are feasible. The sealing material should be capable of maintaining a vacuum, preferably
of approximately 20 millibars, and should not contaminate either the mold compound
or the mold in use. After the edge seal 163 is completed, the entire piece is subjected
to a vacuum. End seals 161 are then positioned and completed while in the vacuum.
The vacuum sealing eliminates air and moisture that would otherwise cause voids in
the molded packages. Alternatives to vacuum sealing may include shrinkwrapping the
package or dipping the mold compound in plastic.
[0046] A critical element to the operation of the molding process using the prepackaged
molding compound is the packaging material. The requirements for the packaging of
the mold compound have been established for an integrated circuit assembly process
using industrial standard requirements for molding compounds and for the resulting
integrated circuit packages. The plastic package should not create residue or glue
like substances in the mold during molding. The mold compound packaging should not
contaminate the mold runners or receptacle. The material used in the packaging should
not add to ionic contamination of the resulting packages, that is the material should
not have an ionic content higher than that of the molding compounds in use in the
integrated circuit packaging art. The material should not melt during the molding
process, so it should have a melting temperature at least ten degrees Celsius over
the molding temperatures. Typically, the material needs to have a melting point of
greater than 200 degrees Celsius.
[0047] Also, the packaging material should only allow the molding compound to exit the package
at selected points adjacent to the mold runners, and it should not open prematurely
during the preheat phase of the molding operation. However, once the edge seals are
penetrated in response to the compression of the mold compound package, the mold compound
should flow out of the package with a minimum of resistance to flow. The material
should not tear in normal handling or shipping, but should have the capacity to stretch
into the runners when compressed during the molding process as described above. The
material should be capable of vacuum sealing and of maintaining the vacuum during
storage.
[0048] Although many materials may exist that could meet these requirements, it is now known
that certain plastic films meet the requirements listed above. Plastic films such
as those used in food storage, freezing and preparation, are particularly well suited
to this application. The melting point, strength, vacuum capability and moisture and
air barrier requirements for the mold compound packaging are all met by such films.
The films are inexpensive and easy to purchase and use in a production environment.
One preferred film is MYLAR™ polyester film, such as for example MYLAR™ 40 XM 963-AT,
a polyester film for packaging available from DuPont, DuPont de Nemours Int. S.A.,
Geneva, Switzerland; or DuPont (U.K.) Ltd, Maylands Avenue, GB-Hemel Hempstead, England.
Another preferred film is ICI™ polyester film. Similar films are commercially available
from a variety of vendors.
[0049] The plastic package 151 of FIG. 9 provides the advantages of making the mold compound
packages impervious to contaminants such as water that could interfere with the molding
process. Since the prepackaged mold compound 155 is self packaged in plastic, storage
and shipping packing materials may be inexpensive and no additional protective layers
are needed. The protection of the mold compound from moisture prevents many of the
package cracking problems and voids associated with moisture contaminated mold compound.
The top 153 may be opaque and may carry labeling information in text and machine readable
forms, such as bar codes or so called UPC labels. This labeling on the mold compound
package 151 provides an easy mechanism for checking that the correct type of mold
compound is being used for a particular packaging operation. Also, the plastic packaging
affords the opportunity to use alternative mold compounds because the mold parts are
not in direct contact with the compound.
[0050] The use of the mold compound insert package 151 with the release films further allows
a variety of compounds to be used in molding, including the resin, resin filled and
epoxy materials of the prior art. Additionally, the use of the package allows the
use of other materials, such as adhesives and abrasives not available in the molding
systems of the prior art.
[0051] Some desirable molding compounds may have poor mold release characteristics, or may
be extremely abrasive. Molding the packages with these compounds in conventional molding
presses can result in packaged units that will not release after molding, or alternatively,
the mold surfaces will wear out much quicker than with conventional molding materials.
The use of the prepackaged mold compound insert combined with the release films eliminates
these problems of the prior art.
[0052] In use, the mold compound is placed in a mold receptacle that is specially shaped
to receive it. A plunger shaped to fit the mold receptacle is used to compress the
mold compound and the plastic package. The mold compound is left in the plastic package
during molding. As the mold compound is compressed, the plastic film is stretched
into the runners and is forced open by the pressure. The plastic film and runners
are preferably positioned so that when the prepackaged plastic package is forced open
the mold compound is forced over the bottom release film 65 within the cavities. In
this way, the defects caused if the mold compound moves under the bottom release film
can be eliminated.
[0053] FIG. 10 depicts the molding system in operation. Top mold 173 and top cavity 69 are
positioned over, and clamped against, bottom mold 175 and bottom mold cavity 63. Top
release film 67 is stretched over and enters into top cavity 69, as described above.
Bottom release film 65 is likewise stretched over and enters into bottom cavity 63,
also as described above. Leadframe 177 and die 189 are positioned in the cavity. The
release films 67 and 65 are clamped together and form a seam at the interface between
the leads of leadframe 177. Prepackaged mold compound insert 179 is compressed by
plunger 181, the plunger tip 182 is beveled to allow the plastic package to compress
more efficiently. The edge seals 183 of the prepackaged mold compound insert 179 open
either in response to heat, pressure or the combination of heat and pressure and the
mold compound is forced out of the plastic package and into the runner 185.
[0054] As the mold compound is forced out, the plastic package is penetrated and forced
open at the runners, and the package elongates into fingers that stretch and extend
into the runner 185. The mold compound is forced along the runner and between the
two release films, as indicated by reference number 180. The release films are positioned
in the runners such that the elongated package fingers direct the mold compound above
the bottom release film 65 in the area labeled 180. Gate 171 then directs the mold
compound into the cavity and between the two release films 67 and 65. The mold compound
fills the cavity formed by the release films 67 and 65 and the top and bottom die
cavities 69 and 63 and encapsulates the die 189 and leadframe 177. After curing, the
mold is opened, the encapsulated leadframe 177 is removed along with the now empty
prepackaged mold compound insert 179. The mold will not require cleaning due to the
use of the prepackaged molding compound and the release films. The release films are
then indexed as described above to prepare for the next molding operation.
[0055] It has further been discovered that the problems of mold compound entering the area
between the bottom release film and the bottom die cavity surface can be reduced if
the mold design and the leadframe design are altered to provide an open gate feature.
This gate design will enhance the molding results when using the release films of
the invention and reduce the probability that the mold compound will erroneously enter
the area underneath the bottom release film. Further, the open gate design is more
flexible than the mold designs of the prior art, as it allows for more alternative
positions of the gate to provide more uniform runner lengths and better mold compound
distribution. The leadframe is designed with an open area where the cavity meets the
gate, to accommodate the release films and the gate design of the mold cavities.
[0056] FIG. 11 depicts a top view of an exemplary dambarless leadframe 191 which incorporates
all the features of the invention described herein. Leadframe 191 has no dambar and
is a 208 pin leadframe for use in producing quad flat package device. In FIG. 11,
a multitude of leads 193 is shown extending away from die pad 195. Leads 193 have
a reduced interlead distance formed in the mold cavity edge area. Straps 199 are used
to provide stability for the die pad 195. Tape 197 is used to provide stability and
to maintain lead planarity during molding. Open gate 198 is used to enable the release
films to receive mold compound during molding, as described above. Leadframe 191 may
be manufactured using etching, stamping or other means as is known in the art. Preferably,
leadframe 191 is plated with palladium and has silver spots in the soldering regions,
to improve solderability.
[0057] FIG. 12 depicts a detailed view of the leads 193 of leadframe 191. The leads stretch
from the wire bond attach area nearest the die pad and then bend and continue on to
form the external leads. Interlead spacers 201 are the widened areas in the mold clamping
region of the leadframe 191. These interlead spacers 201 reduce the spacing 203 between
adjacent leads and also provide a uniform spacing between adjacent leads. As described
above the reduced spacing ensures that the interface formed between the top and bottom
release films during the molding process will prevent the mold compound from leaving
the mold cavity in the areas 203 and forming interlead splinters. Preferably the interlead
spacing in areas 203 is less than 0.20 millimeters, and the best results will be achieved
for typical leadframe thicknesses if the spacing is less than 0.15 millimeters.
[0058] FIG. 13 depicts the details of the stabilization tape 197 used in forming leadframe
191 in FIG. 11. Tape 197 is formed over, and adheres to, each of the leads 193 of
the leadframe. The tape is positioned over, and adheres to, the die pad support strap
199, which is anchored in the corners of the leadframe 191 and provides stability
and support for the die pad 195.
[0059] It has been discovered that a preferred shape for the lead stabilization tape 197
is one that provides uniform distance from the innermost end of the leads 193 to the
inner side of the tape 197. This distance should place the tape as close as is feasible
to the inside of the packaged integrated circuit. By maintaining the distance from
the tape to the inner lead end as a uniform distance, the length of unsupported lead
material 205 is approximately constant around the leadframe. Constant length of the
unsupported lead portions results in a more stable configuration and improved molding
results over the taping supports of the prior art, which, in contrast to the tape
197 of FIGS. 11 and 13, use nonuniform tape to inner lead distances. Nonuniform unsupported
inner lead distance results in variable flexing between the tape and the inner lead
ends, and thus a loss in the planarity of the leadframe during handling and molding
operations. The top view of the stability tape 195 of FIG. 11 shows that when the
inner lead distance is kept uniform, an octagonal shape forms. This may be described
generally as a regular polygon, and is a symmetrical polygon. Other regular polygon
shapes can be used as alternatives to the octagonal shape of the stabilization tape
as shown in FIG. 11, and the benefit of the invention will still be obtained.
[0060] An alternative to the tape of FIGS. 11 and 13 is to form a polymer or epoxy stabilization
bar in the regular polygon shape of the tape of FIGS. 11 and 13. This can be done
when the leadframe is manufactured, by forming the leadframe with an internal tie
bar securing the inner ends of the leadframe together, applying the tape or polymer,
and if required curing can be done to harden a polymer or epoxy. The internal tiebar
is then removed before the leadframe is completed.
[0061] FIG. 14 depicts a detailed view of the gate design of leadframe 191. The open gate
198 is formed in one of the corner instead of the closed gates of the prior art. Tie
strap 199 then couples to the two side supports 196 of the open gate 198.
[0062] This open gate design enhances the operation of the release films and allows the
molding compound to come into the top and bottom of the cavity from a central or open
gate in the mold. The open gate design also allows the gate to be placed in any one
of the four corners, or alternatively it could be placed elsewhere, and it therefore
provides extra flexibility over the designs of the prior art in designing the mold
cavities and the leadframe itself. The mold cavity gate can be above, centered, or
beneath the plane of the leadframe.
[0063] The use of the release films, dambarless leadframe, and prepackaged molding compound
of the invention advantageously enables the packaging of integrated circuits using
fine pitch leadframes without the need for the dambar trim and flash removal processes
of the prior art. The use of the invention enables the production of integrated circuits
using very fine pitch leads and leadframes without expensive and inefficient retooling
of existing assembly facilities. Further, the use of the release films and prepackaged
molding compound inserts of the invention provide the ability to use alternative molding
compounds such as adhesives and high abrasives not previously available.
[0064] While this invention has been described with reference to illustrative embodiments,
this description is not intended to be construed in a limiting sense. Various modifications
and combinations of the illustrative embodiments, as well as other embodiments of
the invention, will be apparent to persons skilled in the art upon reference to the
description.
1. A method for encapsulating integrated circuits, comprising the steps of:
providing a lower cavity region in a lower mold chase;
receiving a mold compound insert in a mold compound receptacle spaced apart from said
lower cavity region;
coupling said mold compound receptacle to said lower cavity region using at least
one runner;
providing an upper cavity region in an upper mold chase, corresponding to said lower
cavity region in said lower mold chase;
positioning a lower release film over the lower cavity region in said lower mold chase;
positioning an upper release film over the upper cavity region in said upper mold
chase;
placing a die assembly on said lower mold chase such that said lower cavity region
receives and supports said die assembly;
clamping said upper mold chase over said lower mold chase such that the upper and
lower release films are brought into contact, the die assembly lying between said
upper and lower release films and within the upper and lower cavity regions;
compressing said mold compound insert and forcing said mold compound out of said mold
compound insert and into the or each runner, until said mold compound transfers into
said runner and substantially fills the area between said upper and lower release
films within said upper and lower cavity regions;
wherein said mold compound is prevented from exiting said upper and lower cavity
regions and forming mold flash between leads of said die assembly by the interface
formed between said upper and lower release films.
2. The method of Claim 1, further comprising the step of stretching said upper and lower
release films into said upper and lower cavity regions using a vacuum.
3. The method of Claim 1 or Claim 2, wherein said step of providing a mold compound insert
comprises providing a mold compound packaged in a plastic film that has a heat seal,
said heat seal becoming penetrable during the molding process, such that the mold
compound exits the plastic package through said heat seal in response to the compression
of said mold compound insert.
4. The method of Claims 1 to 3, wherein said step of providing a mold compound insert
comprises providing a thermoset resin.
5. The method of Claims 1 to 3, wherein said step of providing a mold compound insert
comprises providing a thermoset resin packaged in a plastic film.
6. The method of Claims 1 to 3, wherein said step of providing a mold compound insert
comprises providing a thermoset resin packaged in a plastic film that is heat sealed
at the edges.
7. The method of Claims 1 to 6, wherein said placing step said die assembly comprises;
placing a leadframe and a die on said lower mold chase.
8. The method of Claim 7, wherein said step of placing said die assembly comprise placing
a lead frame having a plurality of leads extending from a die pad positioned inside
the lower cavity region to outer lead tips outside the lower cavity region, each of
said plurality of leads being widened so that the spacing between leads in the area
where the upper and lower release films are clamped together is less than a predetermined
distance.
9. The method of Claim 7 or Claim 8, further comprising the step of providing a leadframe
having a plurality of leads extending from a die pad positioned inside the lower cavity
region to outer lead tips outside the lower cavity region, each of said plurality
of leads being widened so that the spacing between leads in the area where the upper
and lower release films are clamped together is less 0.2 millimeters.
10. The method of Claims 7 to 9, wherein said placing step comprises placing a dambarless
leadframe on said lower mold chase.
11. The method of Claims 2 to 10, wherein said step of receiving a mold compound insert
comprises receiving a mold compound insert in which at least one of said upper release
film and lower release film comprises a plastic film.
12. The method of Claims 2 to 11, wherein said step of receiving a mold compound insert
comprises receiving a mold compound insert in which at least one of said upper release
film and lower release film comprises a flouroplastic film.
13. The method of Claims 2 to 12, wherein said step of receiving a mold compound insert
comprises receiving a mold compound insert in which at least one of said upper release
film and lower release film comprises tetraflouroethylene and ethylene film.
14. The method of Claims 1 to 13, wherein said step of placing said die assembly comprises
placing a lead frame in which each of said plurality of leads is secured to the other
of said leads by a securing tape placed at a position of the leads close to the inner
ends of said leads, said securing tape traversing all of the leads of a particular
leadframe.
15. The method of Claims 1 to 14, further comprising applying a securing tape to each
of the leads of said leadframe such that the distance from the securing tape to the
inner end of each lead is maintained at a substantially constant distance across the
leadframe.
16. The method of Claims 1 to 15, further comprising anchoring said die pad with die straps
coupled to strap anchors positioned at the edge of said leadframe, one of said strap
anchors defining an open area and providing an open gate for receiving the mold compound.
17. A leadframe for use in encapsulating integrated circuits in a mold compound, comprising:
a die pad positioned within said leadframe, and configured to receive an integrated
circuit die;
a plurality of leads each having an inner end near said die pad, and each extending
away from said die pad to an outer lead tip;
securing means for securing each one of said plurality of leads to at least some of
the other ones of said plurality of leads, said securing means being formed close
to the inner ends of said leads;
wherein at least one of said plurality of leads is widened in a region where a
mold cavity will be clamped around said leadframe during an encapsulation process,
such that the spacing between adjacent leads does not exceed a predetermined minimum
distance.
18. The leadframe of Claim 17, wherein said securing means comprises a tape applied around
the plurality of leads near the inner end of said leads, said securing tape having
a center window in the die pad region and forming a regular polygon.
19. The leadframe of Claim 17 or Claim 18, wherein said securing means comprises a polymer
applied to each of the leads near the inner ends of said leads to couple said leads
together.
20. The leadframe of Claims 18 or Claim 19, wherein the regular polygon is an octagon.
21. The leadframe of Claims 17 to 20, wherein each one of said plurality of leads is secured
to at least one other of said plurality of leads.
22. The leadframe of Claims 18 to 21, wherein said spacing between leads is a predetermined
distance which does not exceed 0.20 millimeters.
23. The leadframe of Claims 18 to 21, wherein said spacing between leads is a predetermined
distance which does not exceed 0.15 millimeters.
24. The leadframe of Claims 18 to 23, wherein the leadframe further comprises die pad
straps coupled to strap anchors for supporting said die pad, at least one of said
strap anchors forming an open gate region where no leads are formed, such that during
molding the mold compound may enter the die cavities through said open gate region
of the dambarless lead frame.
25. The leadframe of Claims 17 to 24, wherein said leadframe is dambarless.